Conjugates of nano-diamond and magnetic or metallic particles

ABSTRACT

Techniques for imaging a characteristic of a sample with a plurality of conjugates of diamond-metallic nanoparticles having a nitrogen vacancy center. The plurality of conjugates can be exposed to a sample and the nitrogen vacancy centers can be optically pumped. One or more microwave pulses can be applied to the nitrogen vacancy center, and a fluorescent response can be detected.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent Application No. PCT/US12/048830, filed Jul. 30, 2012, which is related to U.S. Provisional Application Ser. Nos. 61/513,830, filed Aug. 1, 2011, 61/549,046, filed Oct. 19, 2011, 61/562,551, filed Nov. 22, 2011, 61/591,570, filed Jan. 27, 2012, and 61/624,647, filed Apr. 16, 2012, each of which are hereby incorporated herein by reference in their entirety and from which priority is claimed.

STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grant Nos. AFOSR PECASE FA9550-12-1-0045 and FA9550-11-1-0014, awarded by the Air Force Office of Scientific Research. The government has certain rights in the invention.

BACKGROUND

The disclosed subject matter relates generally to techniques for fabricating conjugates of nano-diamond and magnetic or metallic particles for use in imaging and sensing applications.

Fluorescent biomarkers are used in a wide range of applications in biology, chemistry, and other fields. Certain biomarkers can have brightness approximately around an order of magnitude of less than 10̂5 counts/sec, can bleach, blink, or degrade during excitation, and/or are toxic to cells. Optical probes that are non-toxic and efficient and stable light emitters are desirable to sensing in biology and medicine.

Additionally, resolution for imaging with conventional fluorescent biomarkers can be limited by the optical diffraction limit of

${\left. \frac{\lambda}{2\; {NA}} \right.\sim 250}\mspace{14mu} {{nm}.}$

Accordingly, techniques providing resolution below the diffraction limit are also desirable.

SUMMARY

In one aspect of the disclosed subject matter, a method for imaging a characteristic of sample includes providing a plurality of conjugates of diamond-magnetic nanoparticles, one or more of which including a nitrogen vacancy center. The conjugates can be exposed to the sample. The nitrogen vacancy centers can be pumped with a pump light and one or more microwave pulses can be applied. The fluorescent response of the nitrogen vacancy center can then be detected.

In one embodiment, the diamond-magnetic conjugates can include a diamond nanoparticle coupled to a magnetic nanoparticle at a predetermined distance corresponding to a radiative enhancement. Additionally or alternatively, the individual diamond-magnetic conjugates can have a variety of alignments between the magnetic field of the magnetic nanoparticle and the axis of a nitrogen vacancy center in the diamond nanoparticle.

In one embodiment, optically pumping can include directing a continuous wave of pump light. Alternatively, optically pumping can include applying a pulse of pump light prior to applying the at least one microwave pulse and applying an additional pulse of pump light subsequent to applying the at least one microwave pulse.

In one embodiment, detecting a fluorescent response can include detecting emitted photons over an area of the sample. The area of the sample can be divided into a set of pixels. For each pixel, a first, second, and third microwave pulse can be applied and the intensity of a corresponding fluorescent response can be measured. The first frequency can be tuned to a field splitting frequency of the nitrogen vacancy center of one of the conjugates and can correspond to the m_(s)=+1 spin sublevel. The second frequency can be tuned to the zero filed splitting frequency of the nitrogen vacancy center of the nitrogen vacancy center. The third frequency can be tuned to a field splitting frequency of the nitrogen vacancy center of the nitrogen vacancy center. The location of the nitrogen vacancy center can be determined based on the first, second, and third intensities.

In one embodiment, the characteristic of the sample can be magnetic field. For example, a local magnetic field at the location of the nitrogen vacancy center can be determined based on a first, second, and third intensity measurement corresponding to a first, second, and third microwave pulse. Additionally or alternatively, the characteristic of the sample can be pH concentration. For example, the pH concentration can be inferred from, based on the fluorescence response, a determined rate of ionization or reduction of the nitrogen vacancy center. Additionally or alternatively, the characteristic of the sample can be electric field. For example, the electric field can be inferred from, based on the fluorescent response, a determined rate of switching between different charge states of the nitrogen vacancy center. In certain embodiments, the diamond-magnetic conjugates can be bound to a biological molecule.

In another aspect of the disclosed subject matter, a method for fabricating diamond-metal conjugates can include depositing a monolayer of diamond nanoparticles having a predetermined radius on a substrate. The surface of the substrate can be etched to a depth greater than the predetermined radius of the diamond nanoparticles. A layer of metal having a predetermined thickness can be deposited over the monolayer of diamond nanoparticles. The predetermined thickness can correspond to a radiative enhancement rate of a nitrogen vacancy center in a diamond nanoparticle. The metal-covered diamond nanoparticles can be transferred to a second substrate.

In certain embodiments, the metal layer can include gold, silver, or a combination thereof. Additionally or alternatively, the metal layer can include a metallic metal, for example nickel, cobalt, iron, and/or chemical compounds thereof. In one embodiment, the layer of metal can be deposited at an angle.

In another aspect of the disclosed subject matter, a method for fabricating diamond-metal conjugates can include preparing a surface of diamond nanoparticles with an acid treatment. The diamond nanoparticles can undergo silanization with amino-terminated silanes to provide an amine on the surface of the diamond nanoparticles. A surface of the metal nanoparticles can be prepared with a molecule, the molecule having an affinity to bond with the amine. The diamond nanoparticles and metal nanoparticles can be mixed in a solution, whereby the molecule of the surface of the metal nanoparticles can bond to the amine of the surface of the diamond nanoparticles, thereby forming a diamond-metal conjugate.

In one embodiment, the amine is an amine with a length corresponding to a radiative enhancement rate of a nitrogen vacancy center in a diamond nanoparticle. In certain embodiments, the metal nanoparticles can include gold, silver, or a combination thereof. Additionally or alternatively, the metal nanoparticles can include a metallic metal, for example nickel, cobalt, iron, and/or chemical compounds thereof.

In another aspect of the disclosed subject matter, a method for fabricating diamond-metal conjugates can include preparing a surface of diamond nanoparticles with an acid treatment. The surface of at least one of the diamond nanoparticles can be adapted to link to a first end of a DNA strand. Metal nanoparticles adapted to link to a second end of the DNA strand can be provided. The diamond nanoparticles and the metal nanoparticles can be mixed in solution with the DNA strand.

In one embodiment, the DNA strand can have a length corresponding to a radiative enhancement rate of the nitrogen vacancy center of a diamond nanoparticle. In certain embodiments, the metal nanoparticles can include gold, silver, or a combination thereof. Additionally or alternatively, the metal nanoparticles can include a metallic metal, for example nickel, cobalt, iron, and/or chemical compounds thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a nitrogen-vacancy (NV) center in diamond.

FIG. 2 is a block diagram of a conjugate of a diamond nanoparticle and a metallic nanoparticle in accordance with an embodiment of the disclosed subject matter;

FIG. 3 is a flow diagram illustrating a process of fabricating a diamond-metal conjugate in accordance with one embodiment of the disclosed subject matter.

FIG. 4 is a flow diagram illustrating a process of fabricating a diamond-metal conjugate in accordance with another embodiment of the disclosed subject matter.

FIG. 5 is a flow diagram illustrating a process of fabricating a diamond-metal conjugate in accordance with another embodiment of the disclosed subject matter.

FIG. 6 is a flow diagram illustrating a method for imaging a characteristic of a sample in accordance with an embodiment of the disclosed subject matter.

FIG. 7 is a flow diagram illustrating a method for imaging a characteristic of a sample in accordance with another embodiment of the disclosed subject matter.

FIG. 8 is a schematic diagram of a system for imaging a characteristic of a sample in accordance with another embodiment of the disclosed subject matter.

Throughout the drawings, the same reference numerals and characters, unless otherwise stated, are used to denote like features, elements, components or portions of the illustrated embodiments. Moreover, while the disclosed subject matter will now be described in detail with reference to the Figs., it is done so in connection with the illustrative embodiments.

DETAILED DESCRIPTION

Techniques for fabricating conjugates of nano-diamond and magnetic or metallic particles for use in imaging and sensing applications are disclosed herein.

The nitrogen-vacancy (NV) center in diamond can serve as a biomarker. Diamond NV color centers can be formed when a substitutional nitrogen lodges itself in the carbon lattice, replacing two carbons and creating a physical vacancy with dangling bonds. Diamond NV centers can occur naturally or can be implanted in a diamond structure via ion radiation or the like. The NV− center has an additional electron associated with it, creating a desirable electronic S=1 structure that has a long-lived spin triplet in its ground state that can be probed using optical and microwave excitation. The NV electron spin can act as a sensitive probe of the local environment, and their optical accessibility can allow their use in optically-detected magnetic resonance schemes.

Referring to FIG. 1 an exemplary NV center is illustrated. NV centers can absorb photons 140 with a wavelength around 532 nm and emit a fluorescent (PL) response, which can be between 637 and 800 nm. A spin-dependent intersystem crossing 160 between excited state 120 triplet (3) to a metastable, dark singlet level 110 (S) can change the integrated PL for the spin states |0

and |±1

. The deshelving from the singlet 110 occurs primarily to the |0

spin state, which can provide a means to polarize the NVC.

As depicted in FIG. 1, transitions from the NV ground state 110 to the excited state 120 are spin-conserving, keeping m_(s) constant. Such an excitation can be performed using laser light at approximately 532 nm 140; however, other wavelengths can be used, such as blue (480nm) and yellow (580 nm). While the electronic excitation pathway preserves spin, the relaxation pathways contain non-conserving transitions involving an intersystem crossing (or singlet levels).

In accordance with the disclosed subject matter, the NV centers can be used, for example, for voltage imaging. Spin states in the diamond can enable electric and magnetic field sensing on the nanometer scale with high precision at room temperature. Furthermore, the NV centers can be uniquely identified by transitions between their magnetic sublevels, a property that can enable sub-optical spatial resolution. For example, single NVs can be deterministically switched to locate emitters below 30 nm resolution. Diamond nanoprobes with the NV can also be photostable. For example, single NV centers can emit without a change in brightness for months or longer. Additionally diamond is chemically inert, cell-compatible, and has surfaces that can be suitable for functionalization with ligands that target biological samples. NV centers can emit in excess of 10⁶ photons per second, far brighter than certain other light emitters.

In accordance with one aspect of the disclosed subject matter, a diamond nanoparticle including at least one NV center can be coupled to a metal nanostructure to increase the brightness of the emitter. Conjugates of diamond nanoparticles and metallic nanostructures can act as a stable emitter-antenna system. For example, the NV center can be coupled with a larger optical dipole, which can amplify the dipole of the NV center, acting as a nano-antenna.

With reference to FIG. 2 a conjugate of diamond nanoparticles and metallic nanostructures in accordance with an embodiment of the disclosed subject matter will be described. A diamond nanoparticle 200 can contain at least one nitrogen vacancy center 210. The diamond nanoparticle 200 can be coupled to a magnetic nanostructure 220, for example as described below. The diamond nanoparticle 200 can be attached to the magnetic nanostructure 220 at a distance d 230. For example, in an embodiment of the disclosed subject matter, the distance d 230 can correspond to an radiative enhancement of the at least one NV center 210. In some embodiments, the distance d 230 can be predetermined such that the optical dipole of the magnetic nanostructure 220 is located a predetermined distance from the at least one NV center 210.

The conjugate of diamond nanoparticles and metallic nanostructures can be pumped with pump light 240. The pump light 240 can have a wavelength suitable to optically excite the NV center 210. For example, the pump light 240 can have a wavelength of approximately 532 nm. When a photon from the pump light 240 is absorbed by the NV center 210, an electron associated with the NV center 210 can transition to an excited state 120, and then transition back to the ground state 110 and emit a photon 250. The emitted photon 250 can have a wavelength of between 637 nm and 800 nm.

A microwave pulse can be tuned to the field splitting frequency of the nitrogen vacancy center 210 can drive the NV state from the m_(s)=0 state to the m_(s)=±1 state. The can be prepared in the ground state through optical pumping at 532 nm, as a distribution of NV's in various m_(s) sublevels will eventually tend to m_(s)=0. When observing emitted photons, for example with a photon detector, the fluorescence from the zero phonon line (ZPL) and phonon sideband (PSB), relaxation through the singlet levels can reduce observed counts. Thus, applying microwaves tuned to a frequency approximately equal to the zero-filed splitting, D, can cause a drop in the fluorescence from the ZPL. Additionally or alternatively, NV spin state information can be gathered by observing the singlet level transition at 1042 nm. The application of a small magnetic field Bz on the NV-symmetry axis breaks the degeneracy in the ±1 sublevels due to the well-known Zeeman effect.

The diamond nanoparticle 200 can have a diameter between approximately 2 nm and hundreds of nanometers. In an exemplary embodiment, the diamond nanoparticle 200 can have a diameter, for example, between approximately 20 nm and approximately 40 nm. The metallic nanostructrure 220 can have a diameter, for example, between approximately 5 nm and hundreds of nanometers. In an exemplary embodiment, the metallic nanostructure 220 can have a diameter, for example, between approximately 5 nm and approximately 35 nm. In an exemplary embodiment, for example, a gold nanostructure can have a diameter approximately equal to 30 nm, whereby the plasmon resonance corresponds to a radiative enhancement of the diamond NV fluorescence from 640-720 nm. The metallic nanostructure 220 as disclosed herein can be, for example, a non-magnetic particle such as gold or silver. Alternatively, the metallic nanostructure 220 can be a super-paramagnetic particle or other magnetic nanoparticles. For example, the metallic nanostructure 220 can include nickel, cobalt, iron, or chemical compounds thereof, including oxides such as iron oxide.

With reference to FIG. 3, an illustrative method for fabricating hybrid diamond-metal or diamond-magnetic particle single photon emitters can include attaching a metal nanoparticle to a diamond nanoparticle. In one embodiment, a method for fabricating hybrid diamond-metal single photon emitters can include physical deposition of a metal onto the diamond nanoparticles. The metal can be, for example, gold, silver, or a combination thereof. Alternatively, the metal can have magnetic properties and can include, for example, nickel, cobalt, iron, or chemical compounds thereof, including oxides such as iron oxide. A monolayer of diamond nanoparticles, at least some of which having at least one nitrogen vacancy center, having a predetermined radius can first be deposited (310) on a substrate. The surface of the substrate containing the diamond nanoparticles can be etched (320) to a depth greater than the depth of the predetermined radius. A layer of metal can then be deposited (330) up to a predetermined thickness over the monolayer of diamond nanoparticles. The thickness of the layer of metal can correspond to a radiative enhancement rate based on the predetermined radius of the diamond nanoparticles. For example, as noted previously, a 30 nm thick gold layer can produce strong radiative enhancement as the plasmon frequency matches the emission of the NV⁻ center. The diamond-metal conjugates can then be transferred (340) from the first substrate to a second substrate.

The diamond nanoparticles can range from approximately 2 to hundreds of nanometers. Diamond nanocrystals can be produced by high-temperature, high-pressure growth; by detonation; and by fracturing/milling of larger diamonds. They can be purchased commercially and then cleaned using strong acids, as those skilled in the art will appreciate.

Etching (320) can include, for example, reactive ion etching. Reactive ion etching can include the use of, for example, an oxygen or chlorine plasma. For example, reactive ion etching (RIE) can be used with a 10:1 gas mixture of Cl2 and O2 in a Oxford Plasmalab 100 ICP etcher. Using 3 mTorr pressure and DC bias power of 1000 W, with the diamond plate at 60 C during the etching process, etching at 17 nm/second can be realized.

The layer of metal can be deposited (330) by sputter deposition, which is a form of physical vapor deposition in which a metal target can ejected and then deposited onto the target sample. For example, A Kurt J. Lesker Co. PVD75 electron beam evaporator can be operated at 10⁻⁵ torr to evaporate about at a rate of about 0.3-1 nm/sec.

In some embodiments, the layer of metal can be deposited at an angle using a sample holder that is angled with respect to the sputter beam direction.

The thickness of the layer of metal can correspond to a radiative enhancement rate based on the radius of the diamond nanoparticles. The thickness of the metal can be chosen so that the surface plasmon polariton resonance frequency matches the emission spectrum of the NV, e.g., from 630-720 nm. For example, spherical gold particles are can be about 30 nm in diameter for suitable radiative enhancement.

The diamond-metal conjugates can be transferred (340) from the first substrate to the second substrate by pressing an adhesive surface against the surface of the diamond-metal conjugates. For example, the second substrate can be a material with adhesive properties, such as PDMS/silicone. Alternative, a sharp knife edge or abrasive surface can be used to cut off or grind off the nanoparticles.

In another embodiment of the disclosed subject matter, and with reference to FIG. 4, a method of fabricating hybrid diamond-metal single photon emitters can include mixing diamond nanoparticles and metal nanoparticles in a solution. The diamond and metal nanoparticles can be functionalized to bind together at a predetermined distance. This predetermine distance can correspond to a desired radiative enhancement.

The conjugation can be achieved by first preparing (410) the diamond surface through a strong acid treatment, followed by silanization (420) with amino-terminated silanes. The metal particles can be prepared (430) with a molecule that binds to the amine. The two solutions can be mixed together and stirred (440). The length of the amine that is used in this reaction to bind the metal and diamond particles can determine the separation between the two.

In another embodiment, and with reference to FIG. 5, an example method of fabricating hybrid diamond-metal single photon emitters can include attaching a single strand of DNA to a diamond nanoparticle and attaching a second single strand of DNA to a metal or magnetic particle. Diamond nanocrystals can be cleaned with acid (510) to prepare their surface state with carboxyl termination (527). Alternatively, the nanodiamonds can be acid cleaned followed by silanization with amino-terminated silanes (523). The acid can removed by repeated flushing (530) in DI water. Gold or other particles can be purchased with similar surface preparations from companies such as Sigma Aldrich. The DNA sequences can have “sticky” ends, such as NH₂, and can be used to link the surface-treated diamond and metal particles. Suitable DNA strands can be obtained commercially. The metal particles and diamond nanoparticles can be mixed (540) in equal number in a solution with DNA binding strands and agitated (550). The solution can be agitated (550), for example by sonication.

The techniques disclosed herein can provide for bright, photostable atomic defect centers that emit at unique wavelengths ranging from the ultraviolet to the infrared. Emitters coupled to metal particles, as disclosed herein, can emit faster through an enhanced effective electromagnetic dipole, via the metal's surface plasmon polarization mode.

In another aspect of the disclosed subject matter, a method for imaging and tracking individually addressable emitters can include using the conjugates of diamond nanoparticle and magnetic nanoparticles disclosed above.

In one embodiment, the diamond NV can sense the magnetic field of the magnetic particle, which can enable the NV sensor to detect the location and orientation of the magnetic particle. Conjugates of diamond nanoparticles with NV centers coupled to magnetic nanoparticles having a diameter between approximately 5 nm and approximately 35 nm can be provided (610) and exposed (620) to a sample for massively parallel multi-spectral imaging and tracking within liquid environments. For example, the magnetic nano-particle can impart on the NV− electronic ground states a Zeeman splitting. The value of the Zeeman splitting can depend on the precise alignment between the NV− and magnetic nano-particle, and this alignment can be random, allowing for thousands of different values. The splitting can remain the same even as the conjugates tumble in a liquid. Thus, a large number of different Zeeman splittings—on the order of thousands—are available for different diamond-magnet conjugates. These conjugates therefore can correspond to a large number of different colors (in the microwave regime), which can be used to track distinguishable particles under a microscope, with a sub-wavelength resolution. Furthermore, different groups of conjugates can be prepared and functionalized with different compounds, enabling labeling with several thousands of different conjugates that can be optically located to sub-10 nm.

In one embodiment, techniques for imaging a plurality of diamond-magnetic nanoparticle conjugates can include observation (650) of optically detected electron spin resonance (ESR) using microwave transitions (640) within the NV's magnetic sublevels. The NV has a spin-triplet ground state with sublevels corresponding to the spin projections of m_(s)=0,1,−1. The microwave transitions between these levels are on the order of 2.87 GHz in the absence of a magnetic field. When a field is present, such as produced by the magnetic nanoparticle coupled to the diamond, the m_(s)=−1,+1 sublevels can experience an energy shift via the Zeeman effect, which can be proportional to the dot product between the magnetic field and the NV spin axis. This energy shift can be observed (650) by optically detected ESR; from its magnitude, the magnetic field strength can be inferred (660). Moreover, because the magnetic particle's dipole orientation and the NV's orientation are random, a continuous distribution of different Zeeman splittings can be obtained using a plurality of diamond-magnetic nanoparticle conjugates.

An exemplary embodiment in which individual NVs can be uniquely addressed will now be described with reference to FIG. 7 and FIG. 8. However, various modifications will become apparent to those skilled in the art from the following description and the accompanying figures. Such modifications are intended to fall within the scope of the appended claims.

In this embodiment, a confocal scanning technique or emCCD camera technique can be employed. Any suitable emCCD can be employed, for example the ProEM available from Princeton Instruments. In general, a two-dimensional scan of a sample with m NV centers can be given by:

I(x,y,ω)=Σ_(i) ^(m) I _(i)α_(i)[1−D _(i)(P _(RF) ,P _(pump),ω)N _(i)(x,y)],   (1)

where I is intensity, α is collection efficiency; D_(i) is electron spin resonance dips; P_(RF) is power of the radio-frequency field; P_(pump) is the power of the optical 532 nm pump field; ω is the crystal field splitting frequency, and N is a two-dimensional Gaussian distribution with general defining parameters. A confocal scan taken off resonance can be subtracted from a confocal scan taken on resonance to isolate only the photons emitted from the NVs with frequencies ω_(i), given by:

I(x,y,ω _(O))−I(x,y,ω _(i))=I _(i)α_(i) C _(i)(P _(RF) ,P _(pump))N _(i)(x,y)   (2)

D_(i) can be given by:

$\begin{matrix} {{{{Di}\left( {P_{RF},P_{pump},\omega} \right)} = \frac{{C_{i}\left( {\gamma_{i}/2} \right)}^{2}}{\left( {\gamma_{i}/2} \right)^{2} + \left( {\omega - \omega_{i}} \right)^{2}}},} & (3) \end{matrix}$

where C_(i)i(P_(RF),P_(pump)), γ_(i)(P_(RF),P_(pump)), and ω_(i)=ω_(O)±{right arrow over (B)}·{right arrow over (μ)}_(i). N_(i)(x,y) can be given by:

$\begin{matrix} {{N_{i}\left( {x,y} \right)} = {\frac{^{\frac{- 1}{2{({1 - \rho_{i}^{2}})}}}\begin{bmatrix} {\frac{\left( {x - \mu_{ix}} \right)^{2}}{\sigma_{ix}^{2}} + \frac{\left( {x - \mu_{iy}} \right)^{2}}{\sigma_{iy}^{2}} -} \\ \frac{2\; {\rho_{i}\left( {x - \mu_{ix}} \right)}\left( {x - \mu_{iy}} \right)}{\sigma_{ix}\sigma_{iy}} \end{bmatrix}}{2\; \pi \; \sigma_{ix}\sigma_{iy}\sqrt{1 - \rho_{i}^{2}}}.}} & (4) \end{matrix}$

The precision with which the two-dimensional position of the NV center can be known can increase with the square root of the integration time due to the poissonian nature of the source. Such precision can be limited by the environment, including piezoelectric drift of a sample holder and thermal fluctuations throughout the setup.

The Zeeman splitting of an NV center can be measured by using optically detected magnetic resonance (ODMR). For example, a continuous wave of green pump laser can be applied for polarizing the spin of the NV into the m_(s)=0 ground state, and a microwave field in the GHz regime can resonantly excite the NV from the m_(s)=0 ground state to the m_(s)=±1 ground states. Due to the bias of the m_(s)=±1 excited states to decay into a metastable singled state with a longer life (e.g., 300 ns) than the lifetime of the m_(s)=0 excited state (e.g., 10 ns), the fluorescence into the 637-800 nm band can decrease.

A plurality of conjugates of diamond magnetic nanoparticles can be provided (710). One or more of the conjugates can have a nitrogen vacancy center within its diamond nanoparticle. The plurality of conjugates can be exposed (720) to a sample 820. The sample 820 can be a fluid, such as a biological fluid or a substrate in the solid state, such as a circuit. In some embodiments, the conjugates can be exposed to a sample within a tissue. For example, the conjugates can be introduce into a cell or into a lumen.

The conjugates can be optically pumped (730) to excite the nitrogen vacancy centers contained therein. For example, the conjugates can be continuously pumped with green laser at approximately 532 nm with a power near 1 mW focused to a 500 nm spot. For pulsed excitation, the power can scale down with the duty cycle. In some embodiments, optical pumping can occur at discrete times. For example, a first pulse of pump light can be applied to drive the NV spin states into the m_(s)=0 sublevel. An additional pulse of pump light can be applied for readout.

Optical pumping (730) can be accomplished with a suitable light source 810, which can include a green laser capable of emitting light at 532 nm. Addition optics 815 and 835 can be employed to guide, filter, focus, reflect, refract, or otherwise manipulate the light. Such optics can include, for example, a pinhole aperture and/or barrier filter (not shown). Additionally, a dichromatic mirror 840 can be used to direct pump light to the sample 820 while transmitting a PL response. For example, the light source 810 can be arranged such that pump light 811 is reflected off of a dichromatic mirror 840 and towards the sample 820. A PL response from the sample will be directed through the dichromatic mirror 840 in a direction orthogonal to the orientation of the light source 810.

The pump light 811 can be directed through an objective 850 to the sample 820. Photons in the pump light 811 can be absorbed by the NV centers within the conjugates exposed to the sample 820, thereby exciting the NV center into an excited state. The NV can then transition back to the ground state, emitting fluorescent response 812, e.g., a photon with a wavelength between 637 and 800 nm. This fluorescent response can pass through the objective 850 and the dichromatic mirror 840 to a photodetector 830. In certain embodiments, the photodetector 830 can include a photomultiplier. The photodetector 830 can be, for example, an emCCD camera. Alternatively, the photodetector 830 can be a scanning confocal microscope or other suitable photon detector.

In certain embodiments, the axis of the nitrogen vacancy centers in the diamond nanoparticle of the conjugate can be aligned random angles with the magnetic field of the magnetic nanoparticle. The magnetic nano-particle can impart on the NV− electronic ground states a Zeeman splitting between the m_(s)=+1 and a m_(s)=+1 spin sublevels. The value of the Zeeman splitting can depend on the precise alignment between the NV− and magnetic nano-particle, and this alignment can be random, allowing for thousands of different values.

The area of the sample 820 can be divided into a number of pixels, each pixel corresponding to subset of the area. For each pixel, the fluorescent response 812 can be measured for various microwave pulses. In this manner, the location of a single NV center can be addressed due to the particular Zeeman splitting resulting from the precise alignment between the magnetic nanoparticle and the NV center. For example, a first microwave pulse can be applied (740) at a first frequency ω₊₁. This first frequency can be tuned to the field splitting frequency of a single NV center based on its Zeeman splitting. This microwave pulse can drive the single NV center into the m_(s)=+1 state, which can result in lower resulting fluorescent intensity due to transition through the shelving state. However, this pulse will not drive any surrounding NV centers into the m_(s)=+1 state because different orientations of the their corresponding magnetic nanoparticles will result in different Zeeman splitting. The photodetector 830 can then measure (750) the intensity of the fluorescent response.

A second microwave pulse can be applied (760) at a second frequency ω₀. The second frequency can be tuned to the zero field splitting frequency of a single NV center, e.g., 2.87 GHz. The photodetector 830 can then measure (770) the intensity of the fluorescent response. In like fashion, at least a third microwave pulse can be applied (780) at a third frequency ω⁻¹. The third frequency can be tuned to the field splitting frequency of a single NV center based on its Zeeman splitting. This microwave pulse can drive the single NV center into the m_(s)=−1 state, which can result in lower resulting fluorescent intensity due to transition through the shelving state. The photodetector 830 can then measure (790) the intensity of the fluorescent response.

The location of the single NV center can then be determined (799) based on the intensities of the fluorescent responses corresponding to the first, second, and third microwave pulses. For example, the control unit 890, which can include a processor and a memory, can subtract the intensities corresponding to frequencies ω₊₁ and ω⁻¹ from the intensity corresponding to ω. Additional processing techniques can be employed to further reduce uncertainty regarding the position of the NV center.

Each pixel can then be combined and displayed, thereby providing a graphical representation or image of the location of one or more NV centers. The techniques disclosed herein can be repeated for a plurality of NV centers, which can allow for tracking of individual NV centers.

In one embodiment, the diamond-magnet nanoparticle conjugates can be coupled with a protein for biological imaging. The binding can employs surface functionalization of the nanodiamond, for example via carboxyl groups or silanization with ammene groups. Carboxyl or ammene groups can be, for example, paired with DNA or antibodies for selective binding to a particular kind of protein to be studied. Once the diamond-magnet is linked to the protein, optical electron spin techniques, as disclosed herein, can be used to measure properties of the protein, such as its chemical composition or oxidative state. The chemical composition can be inferred by magnetic resonance imaging of its constituent atoms. The oxidative state can be inferred from spin noise due to a free electron, or oxidation-state-dependent paramagnetic/diamagnetic state of certain molecules such as hemoglobin. The magnetic nanoparticle in the diamond-magnet conjugates can produce a sharp magnetic field gradient that causes different Zeeman splittings of protein molecules as a function of their position within the magnetic field. Additionally, the Zeeman splitting between the magnetic particle and NV can be used to uniquely identify a particular NV-nanodiamond-protein conjugate through the electron spin signal, which can allow for the tracking of a single conjugate within a cell or other fluid environment, or to address it using super-resolution imaging, for example as disclosed above.

In one embodiment, the diamond-magnet nanoparticle conjugates can be used for high resolution imaging of a magnetic field. The magnetic particle-NV conjugate can have a unique ground state Zeeman splitting that can enable super-resolution imaging using deterministic emitter switch microscopy, as disclosed herein. In addition, the response of the magnetic nanoparticle can be used to more precisely measure the local magnetic field environment, as it can act as an amplifier of the magnetic field. The magnetic field can then be determined using the Zeeman splitting of the NV magnetic sublevels.

In one embodiment, the diamond-magnet nanoparticle conjugates can be used for high resolution imaging of electric field and pH. pH concentration can be measured by the rate of reduction/ionization of an NV that is located approximately 2-30 nm below the diamond surface. The reduction and ionization can produces different NV charge states, including NV²⁺, NV⁺ NV⁰, and NV⁻. These different charge states can be associated with different emission spectra. By measuring the rate of switching between different charge states and the mean occupation of these charge states, one can infer information about the local charge electric field and the local charge environment and abundance of radicals.

The presently disclosed subject matter is not to be limited in scope by the specific embodiments herein. Indeed, various modifications of the disclosed subject matter in addition to those described herein will become apparent to those skilled in the art from the foregoing description and the accompanying figures. Such modifications are intended to fall within the scope of the appended claims. 

1. A method for imaging a characteristic of a sample, comprising: providing a plurality of conjugates of diamond-magnetic nanoparticles, one or more of the plurality of conjugates including a diamond nanoparticle having a nitrogen vacancy center; exposing the plurality of conjugates to the sample; optically pumping the nitrogen vacancy center; applying at least one microwave pulse to the nitrogen vacancy center; and detecting a fluorescent response of the nitrogen vacancy center.
 2. The method of claim 0, wherein the providing further comprises providing at least one diamond nanoparticle bonded to a magnetic nanoparticle by a predetermined distance, the predetermined distance corresponding to a radiative enhancement.
 3. The method of claim 0, wherein optically pumping includes directing a continuous wave of pump light at approximately 532 nm to the nitrogen vacancy center.
 4. The method of claim 0, wherein optically pumping includes applying a pulse of pump light at approximately 532 nm to the nitrogen vacancy center prior to applying the at least one microwave pulse, and applying a pulse of pump light at approximately 532 nm to the nitrogen vacancy center subsequent to applying the at least one microwave pulse.
 5. The method of claim 0, wherein the providing further comprises providing a plurality of conjugates including a nitrogen vacancy center having a different orientation with respect to a corresponding magnetic nanoparticle relative to other conjugates, whereby a component of a magnetic field of the magnetic nanoparticle is exerted along an axis of the nitrogen vacancy center, and whereby a m_(s)=+1 and a m_(s)=+1 spin sublevel of the nitrogen vacancy center experience Zeeman splitting.
 6. The method of claim 5, wherein detecting a fluorescent response further includes detecting emitted photons over area of the sample, the area divided into a set of pixels each corresponding to a subset of the area.
 7. The method of claim 6, wherein detecting emitted photons includes, for each pixel: applying a first microwave pulse at a first frequency, the first frequency tuned to a field splitting frequency of the nitrogen vacancy center of one of the plurality of conjugates, the field splitting frequency corresponding to the m_(s)=+1 spin sublevel; measuring a first intensity of a first fluorescent response based on the first frequency; applying a second microwave pulse at a second frequency, the second frequency tuned to a zero field splitting frequency of the nitrogen vacancy center of the one of the plurality of conjugates; measuring a second intensity of a second fluorescent response based on the second frequency; applying at least a third microwave pulse at a second frequency, the third frequency tuned to a field splitting frequency of the nitrogen vacancy center of the one of the plurality of conjugates, the field splitting frequency corresponding to the m_(s)=−1 spin sublevel; measuring at least a third intensity of a third fluorescent response based on the second frequency; determining a location of the nitrogen vacancy center based on the first, second and third intensities.
 8. The method of claim 7, wherein determining the location includes subtracting the first and second intensities from the first intensity.
 9. The method of claim 7, wherein the characteristic of the sample is magnetic field and further comprising determining a local magnetic field a the location of the nitrogen vacancy center based on the first, second, and third intensities.
 10. The method of claim 0, wherein the providing further comprises providing one or more conjugates bound to a biological molecule.
 11. The method of claim 0, wherein the characteristic of the sample is pH concentration, and further comprising determining, based on the fluorescent response, a rate of ionization or reduction of the nitrogen vacancy center.
 12. The method of claim 0, wherein the characteristic of the sample is electric field, and further comprising determining, based on the fluorescent response, a rate of switching between different charge states of the nitrogen vacancy center.
 13. A nanoparticle diamond-metal conjugate prepared by the method, comprising: depositing a monolayer of diamond nanoparticles having approximately a predetermined radius on a substrate, one or more of the diamond nanoparticles having a nitrogen vacancy center; etching the surface of the substrate to a depth greater than the predetermined radius; depositing a layer of metal having a predetermined thickness over the monolayer of diamond nanoparticles, the predetermined thickness corresponding to a radiative enhancement rate of the nitrogen vacancy center based on the predetermined radius of the diamond nanoparticles; and transferring the metal-covered diamond nanoparticles to a second substrate.
 14. The nanoparticle diamond-metal conjugate of claim 13, wherein the depositing a layer of metal comprises depositing a metal selected from the group consisting of gold, silver, or a combination thereof.
 15. The nanoparticle diamond-metal conjugate of claim 13, wherein the depositing a layer of metal comprises depositing a magnetic layer.
 16. The nanoparticle diamond-metal conjugate of claim 15, wherein the depositing a layer of metal comprises depositing a metal selected from the group consisting of nickel, cobalt, iron, or chemical compounds thereof.
 17. The nanoparticle diamond-metal conjugate of claim 13, wherein depositing a layer of metal includes depositing the layer of metal at an angle.
 18. A nanoparticle diamond-metal conjugate prepared by the method, comprising: preparing a surface of diamond nanoparticles with an acid treatment, one or more of the diamond nanoparticles having a nitrogen vacancy center; treating the diamond nanoparticles with one or more amino-terminated silanes to provide an amine on the surface of the diamond nanoparticles; preparing a surface of metal nanoparticles with a molecule, the molecule having an affinity to bond with the amine; and mixing the diamond nanoparticles and metal nanoparticles in a solution, whereby the molecule of the surface of the metal nanoparticles bonds to the amine of the surface of the diamond nanoparticles, thereby forming a diamond-metal conjugate.
 19. The nanoparticle diamond-metal conjugate of claim 18, wherein the treating further comprises selecting an amine with a length corresponding to a radiative enhancement rate of the nitrogen vacancy center.
 20. The nanoparticle diamond-metal conjugate of claim 18, wherein the preparing further comprises preparing a surface of metal nanoparticles selected from the group consisting of gold, silver, or a combination thereof.
 21. The nanoparticle diamond-metal conjugate of claim 18, wherein the preparing further comprises preparing a surface of magnetic nanoparticles.
 22. The nanoparticle diamond-metal conjugate of claim 21, wherein the preparing further comprises preparing a surface of metal nanoparticles selected from the group consisting of nickel, cobalt, iron, or chemical compounds thereof.
 23. A nanoparticle diamond-metal conjugate prepared by the method using a DNA strand, comprising: preparing a surface of diamond nanoparticles with an acid treatment, one or more of the diamond nanoparticles having a nitrogen vacancy center; adapting the surface of at least one of the diamond nanoparticles to link to a first end of the DNA strand; providing metal nanoparticles, at least one of the metal nanoparticles adapted to link to a second end of the DNA strand; mixing the diamond nanoparticles and the metal nanoparticles in solution with at least the DNA strand.
 24. The nanoparticle diamond-metal conjugate of claim 23, wherein the DNA strand comprises a DNA strand having a length corresponding to a radiative enhancement rate of the nitrogen vacancy center.
 25. The nanoparticle diamond-metal conjugate of claim 23, wherein the providing further comprises providing metal nanoparticles selected from the group consisting of gold, silver, or a combination thereof.
 26. The nanoparticle diamond-metal conjugate of claim 23, wherein the providing further comprises providing magnetic nanoparticles.
 27. The nanoparticle diamond-metal conjugate of claim 26, wherein the providing further comprises providing metal nanoparticles selected from the group consisting of nickel, cobalt, iron, or chemical compounds thereof. 